In my opinion, the best thing about Irrlicht is the learning curve. I was literally programming 3D applications after I had examined the "hello, world" sample program. It uses a class-based system that is intuitive and easy to remember. It is also simple, and much of the complexities are handled by the engine itself, rather than making the programmer responsible.
Irrlicht supports a wealth of file formats. It will load and display 3ds Max files, Quake 2 MD2 Models, Maya .obj objects, Quake 3 .bsp maps, Milkshape3D objects, and DirectX .x files. 3ds Max and Maya objects are widely supported by many 3D modelers (such as Wings3D), so creation of these files is very easy.
Particle systems are also implemented in the engine, making creation of complex effects such as fire and fountains of water very easy. It even has a rudimentary gravity effect that you can attach to particle systems that cause the particles to fall after a few seconds.
Another nifty feature is the concept of "animators." Animators are classes that can be attached to any object within your program, and are primarily involved in smooth movement of 3D objects. They are easily extended to fit your needs. For example, I had no trouble modifying the "FlyCircleAnimator" to cause the object to fly on an oval-shaped path.
Irrlicht is also well-documented. Although its author, Nikolaus Gebhardt, is slightly rusty on some of his English, the API reference is by no means unreadable. For some of the inner workings of the engine, there is no documentation, but most of the undocumented classes are fairly self-explanatory.
The engine has one shortcoming that I find annoying: It is virtually impossible to create Irrlicht-compatible animated models without high-end software. The animated model formats that Irrlicht supports are MD2, the Quake 2 character model format, and Microsoft's own .x format. I found creation of models in these formats to be cumbersome at best. I plunked down $25 for Chumbalumsoft's Milkshape3D, but found even that to be lacking in model creation capabilities. It would seem that the best way to design .x models is to model in 3ds Max and use Microsoft's exporter. However, this is but wishful thinking for those of us with limited resources (3ds Max costs $4,000).
Also, as of version 0.6, the "skybox" object is only capable of displaying 16-bit textures, thus showing color banding on higher-quality images. I find this to be a nuisance.
When using the Direct3D renderer, the engine will cause the program to crash if it's running in windowed mode and the user's color bit depth is not 16 or 32 bits. However, this is easily overcome with some basic error checking.
While Irrlicht is clearly still in the beta stadium, it shows some real promise. It has a large, active community that can (and will) answer questions about it.
Irrlicht gave me the motivation to learn C++, as well as teaching me many programming skills. The entire package is only 10MB, so I encourage you to download it and give it a shot! Both Windows and Linux are supported, but Direct3D is only available for Windows (naturally). I have not tried using Irrlicht under Linux, so your mileage may vary.